1. Field of the Invention
This invention relates to zoom lenses of small size, and more particularly to zoom lenses of shortened total length (from the front vertex to the image plane) suited to lens-shutter cameras, video cameras, or the like.
2. Description of the Prior Art
Recently, decreases in size of lens-shutter cameras, video cameras, and the like, have created a demand for reductions in the overall lengths of zoom lenses. Also, cameras whose lenses are not interchangeable, such as lens-shutter cameras, often use zoom lenses, and it would be desirable to shorten the total length of such zoom lenses to a value almost equal to that of the hitherto used fixed focal length lenses.
For this purpose, a proposal has been made for a two-unit zoom lens composed, as shown in FIG. 1, of a front lens unit I of positive power and a rear lens unit II of negative power. The separation between units are varied to vary the image magnification. This appears in Japanese Laid-Open Patent Application No. SHO 57-201213.
According to this proposal, the use of positive and negative powers in this order from the front provides a good compromise between the requirements of shortening the lens' back focal distance and of simplifying the construction of the operating mechanism.
Another proposal appears in Japanese Laid-Open Patent Application No. SHO 58-184916. Here, three lens units of positive, positive, and negative refractive powers in sequence from the front form a zoom lens, and all the three lens units are made movable for varying the image magnification.
The above-described two-component zoom lens suffers from the fact that there are only two movable lens units and the total zoom movement must be made relatively long to obtain a desired zoom ratio. In the three-component zoom lens, on the other hand, the three lens units are moved to zoom the lens. However, here the structure of the operating mechanism tends to become complicated.
Another problem common to both of the two- and three-component zoom lenses is that the simultaneous axial movement of the diaphragm during zooming calls for an increase in the complexity of structure of the operating mechanism.
The purpose of reducing the size of the lens is mainly to minimize the bulk and size of the camera as a whole. Yet, a much-desired reduction in the size of the lens may cause an increase in the size of the lens's mounting and its operating mechanism and, therefore, in the diameter and length of the lens's outer barrel. Thus, the lens's size reduction may be meaningless. That is, in reducing the size of the lens, one must consider its influence on the structure of its mechanical mount.